Fix -Wwrite-strings warning in non-Pango wxFontEnumerator code

Don't convert wxString::c_str() to decltype(wxT("*")), which is
non-const "wchar_t *". Just pass wxString to Printf() directly instead,
this is simpler and more efficient anyhow.

src/unix/fontenum.cpp

@@ -282,7 +282,7 @@ bool wxFontEnumerator::EnumerateEncodings(const wxString& family)
 #else
     wxString pattern;
     pattern.Printf(wxT("-*-%s-*-*-*-*-*-*-*-*-*-*-*-*"),
-                   family.empty() ? wxT("*") : family.c_str());
+                   family.empty() ? wxString("*") : family);
 
     // get the list of all fonts
     int nFonts;